How to go home from Duna with 478 m/s delta v?

[TaxiService]

Hi people,

I have made a poor 1.0 decision. I shouldn't put two nuke rockets on the Duna visit craft. Now I have only 478 m/s delta v after landing on Duna and returning back to the orbiting rocket. I need about 650 m/s delta v to get the orange dudes home safely.

Now I have three options of accomplishing it.

1) Use gravity assist of Ike of 300 m/s orbital velocity to accelerate for free, with current craft setup (both vehicles)

2) Undock and park the heavy lander in orbit to gain the total 818 m/s delta v to send the nuke+hitchhiker rocket home directly. But I am using Remote Tech mod (no remote-control in middle of home path), and would have no correction-control between Duna and Kerbin.

3) Send a refuel vehicle to Duna on next window. The wait will be like 1.0-1.5 years.

The question is I am trying to figure out how to use the Ike gravity assist correctly. See the second photo in the second spoiler. Is A path the correct way?

On a happy note, I applied my lesson on designing a EvE craft (just visiting not landing) with a single nuke and mod liquid-only tank.

Edited July 15, 2015 by TaxiService

[Randazzo]

Hi people,

I have made a poor 1.0 decision. I shouldn't put two nuke rockets on the Duna visit craft. Now I have only 478 m/s delta v after landing on Duna and returning back to the orbiting rocket. I need about 650 m/s delta v to get the orange dudes home safely.

Now I have three options of accomplishing it.

1) Use gravity assist of Ike of 300 m/s orbital velocity to accelerate for free, with current craft setup (both vehicles)

2) Undock and park the heavy lander in orbit to gain the total 818 m/s delta v to send the nuke+hitchhiker rocket home directly. But I am using Remote Tech mod (no remote-control in middle of home path), and would have no correction-control between Duna and Kerbin.

3) Send a refuel vehicle to Duna on next window. The wait will be like 1.0-1.5 years.

http://i.imgur.com/s5VHcaJ.jpg

http://i.imgur.com/zhaXO8P.png

The question is I am trying to figure out how to use the Ike gravity assist correctly. See the second photo in the second spoiler. Is A path the correct way?

On a happy note, I applied my lesson on designing a EvE craft (just visiting not landing) with a single nuke and mod liquid-only tank.

http://i.imgur.com/JkOnIQM.jpg

Considering that you have to brake on the other end or get flung out into a Solar orbit, I'd say you don't make that trip and send a refueling mission. Consider that Duna is higher than Kerbin in it's orbit, and therefore you will be accelerating the whole way. I also don't see any heat shielded pods on that ship, so aerobraking would be dangerous.

Admittedly, I don't even aerobrake coming back from the Mun or Minmus unless I absolutely have to, so I'm not much of an expert on that. I can't imagine it'd be safe to smack into the atmosphere coming back from another planet, but this is KSP.

Alternatively you could take the science from the lander and store it and your kerbals in the hitchhiker container (I think you can do that... maybe..?), ditch the lander, and if that provides enough of a boost to get back and capture into orbit around Kerbin, send up another vehicle to recover the crew and science. This is, of course, assuming there is space in that container.

[Snark]

...So you're actually orbiting Duna retrograde? i.e. westwards, opposite the direction of Duna's spin, Ike's orbit, and Duna's orbit around the sun? Ew, that makes life harder. Just out of curiosity, how did that happen? Anyway, water under the bridge at this point.

I assume that the big red arrow that you've drawn from Duna indicates its orbital direction around the sun?

You want your ejection from the Duna/Ike system to be opposite Duna's orbital direction, in order to lower your solar periapsis down to Kerbin's orbit. The fact that you're orbiting opposite Ike's direction makes an Ike gravity assist considerably less effective. Offhand I'd say that if you do any Ike assist, you'd want option B, since that gives you the maximal boost opposite Duna's orbital direction.

However, it's not at all clear to me that you get that much of a benefit (or indeed any benefit at all)-- it would mean giving up a fair amount of Oberth-effect benefit from Duna's gravity well, which is deeper than Ike's.

Some more options to consider:

Personally, I like the idea of ditching the command pod and sending them home in the hitchhiker module. Yes, it means no orbital corrections between Duna and Kerbin. However, a well-executed burn from Duna should have no problem taking you close in to Kerbin where you will have control again (especially if you don't play around with Ike).

You could also consider a hybrid approach between "send them home in the hitchhiker" and "send a rescue mission": that is, send them home in the hitchhiker with the best burn you can manage from Duna, then launch a "rescue" mission that intercepts them before they get to Kerbin. The rescue mission wouldn't even have to match velocity or dock-- it could whizz past at kilometers per second at a distance of a few thousand km, and simply serve as a RemoteTech relay that would give you control over the ship long enough to do a correction burn. It could be small, cheap, unmanned, and (because really light) have a lot of dV.

In the it's-so-crazy-it-just-might-work department: You could send them home on EVA. A kerbal's EVA pack has a ridiculously overpowered dV of several hundred m/s. Launch them in the ship, then when you run out of rocket fuel, they go EVA and complete the boost. Yes, it would be a hassle (you'd have to do it separately for each kerbal, and you won't have navball or maneuver nodes)... but they'd live forever in legend and song!

[Sharpy]

Skirting Kerbin atmosphere at 52km won't kill you and while it may not be enough for aerocapture, it should make enough of difference to allow capture within Kerbin's SOI when braking at periapsis with whatever fuel you have left. Just make sure to make as good Kerbin encounter as you can before losing contact, so that you could cheaply correct for the right periapsis once you restore control. And once captured with atmospheric periapsis, landing is just a matter of time.

(...or maybe send out a relay probe to provide relay for your guys somewhere 2 weeks from Kerbin, so that you could perform the correction then.)

"A" path is the correct accelerating path.

Also, before ditching the lander, use up the rest of monoprop for a few more km/s before the proper escape burn. And plan four nodes: 1. Escape Duna, get Ike flyby. That should take most of your delta-V; Oberth effect at Duna low orbit will be much stronger than at Ike flyby. 2. Get a perfect encounter with Ike, 3. get the right escape from Ike (at periapsis) 4. once out of Ike SOI perfect your Kerbin encounter.

Edited July 14, 2015 by Sharpy

[mhoram]

From how I understand gravity assists, it is the following:

- Using a type "A" gravity assist typically increases your orbital energy when you travel prograde

- Using a type "B" gravity assist typically lowers your orbital energy when you travel prograde

Since you travel retrograde, it becomes difficult to say, if A or B are better. Try plotting nodes and see which one boosts your orbit more.

The biggest problem I see is timing.

Using http://ksp.olex.biz/ you can get an estimation where Ike should be while you are within it's SOI (ejection angle ~ 96°)

Since you travel retrograde this can not be applied!

Then you time your low-Duna-Burn so that you reach Ike while it is at the stated angle - and try to plan the burn so that after the SOI transitions Duna->Ike->Duna->Kerbol you reach Kerbin.

The amount of dV for the low-Duna-burn you have to find by experimenting with the maneuver node.

Optinally a correction-burn at Periapsis while within Ike's SOI might be necessary.

Depending on the orbit after leaving Ike's SOI, it might be necessary to adjust the location where you meet Ike on it's orbit: your periapsis (around Duna) should be at the ejection angle and it's altitude should match the parking orbit altitude.

Never tried such a maneuver before however - seems like a lot of detail-work is needed to make this work.

Edited July 14, 2015 by mhoram
corrected statement

[Sharpy]

...wait. Both paths are wrong.

"B" path is pointless as it will just reduce your speed relative to Duna and force you to waste more more fuel on escape.

"A" path accelerates you, but in the wrong direction - raising your Sun apoapsis instead of lowering it.

Move Ike 180 degrees so that it's moving towards the back of Duna's path. Then apply "A" style path (flyby "behind" Ike) but escaping Duna in the opposite direction than where it's moving. When it comes to escape, retro/prograde orbit really doesn't matter much, so don't sweat it.

Actually, move it way more than 180 degrees. Ike's gravity is not nearly enough to twist your path so much - unless you are moving way too slow. Great most of your escape velocity should be gained at Duna's low orbit. Only burn at Ike periapsis for corrections; the Oberth effect will be waaay stronger at Duna - Ike gives you just the free gravity assist.

[Edax]

Hi people,

I have made a poor 1.0 decision. I shouldn't put two nuke rockets on the Duna visit craft. Now I have only 478 m/s delta v after landing on Duna and returning back to the orbiting rocket. I need about 650 m/s delta v to get the orange dudes home safely.

http://i.imgur.com/z26GsRG.jpg

Now I have three options of accomplishing it.

1) Use gravity assist of Ike of 300 m/s orbital velocity to accelerate for free, with current craft setup (both vehicles)

2) Undock and park the heavy lander in orbit to gain the total 818 m/s delta v to send the nuke+hitchhiker rocket home directly. But I am using Remote Tech mod (no remote-control in middle of home path), and would have no correction-control between Duna and Kerbin.

3) Send a refuel vehicle to Duna on next window. The wait will be like 1.0-1.5 years.

http://i.imgur.com/s5VHcaJ.jpg

http://i.imgur.com/zhaXO8P.png

The question is I am trying to figure out how to use the Ike gravity assist correctly. See the second photo in the second spoiler. Is A path the correct way?

On a happy note, I applied my lesson on designing a EvE craft (just visiting not landing) with a single nuke and mod liquid-only tank.

http://i.imgur.com/JkOnIQM.jpg

The safest and most simplest method would just to send a refueling probe. Docking ports, plenty of airbrakes, large heatshield (doesn't need that much ablator) and make it sturdy, and just hurl it at Duna at high speed without waiting for the transfer window, and aerobrake hard. And double check to see if the resupply module will enter in a retrograde orbit so it can link up.

Edited July 14, 2015 by Edax

[Tatonf]

This is a bit sad because you just need a bit more to get the full ship home (550 m/s with a burn at periapsis).

I tried some Ike slingshoot but the fact that your orbit is retrograde AND inclined by 15° doesn't help at all. Most of the time you end up with orbits that makes your ship crash on Duna or put you into an even more inclined orbit.

I got some eliptical orbits that would have a periapsis at 60 km for better Oberth effect and a pretty high apoapsis. Problem is, you would need to wait an unknow numbers of windows before the apoapsis get ligned up correctly so a burn at periapasis would get you home with a Hohmann transfer.

Or you could go for some Duna gravity assist.

But I got bored with that so good luck.

[Superfluous J]

What's your dV if you toss that lander and leave it in orbit?

What's the dV of the lander if you put all the fuel in it and leave the "transfer stage" behind?

You have 3 guys and seating for 8, toss away some of that extra space and you may have the dV you need.

Also, you can avoid sending a refueller AND aerobraking by getting close to Kerbin with the ship, and sending up a ship that can rendezvous with it (like an Asteroid rendezvous), dock with the open port that you have by leaving the other half of the Duna ship behind, and then slow it down. Or, just transfer the guys and their science over and bring them home in the more capable vessel.

[peachoftree]

If you do option 2, you can get a super accurate encounter using alexmoons transfer calculator and precise node. when you click on the i next to the delta-v in the calculator, it will give you values you can plug in to precise node and only require very minimal tweaking to get an encounter with kerbin. It is how I do all my encounters.

[Snark]

What's the dV of the lander if you put all the fuel in it and leave the "transfer stage" behind?

He can't do that, because there's practically no oxidizer available. All his fuel is just LF with no O, which means the LV-Ns are his only option. So all he could do is either try to take the whole shebang, or ditch the lander. Going home in the lander by itself just isn't an option.

It sounds as though he has plenty of dV if he ditches the lander, the only disadvantage being that then he wouldn't be able to control his ship for most of the journey: as soon as he departs Duna's neighborhood, he's incommunicado and therefore uncontrollable (because RemoteTech) until he gets right near Kerbin, which means no mid-course correction burn possible.

My money would be go for it. Set up a Duna departure burn that takes the nuke ship right to Kerbin, and just live with the uncontrollable middle part of the journey. If a mid-course is really needed, just send a rescue craft with a relay antenna on it that gets close enough to give control of the ship long enough to make a correction burn.

[bzimac]

This is really a "Houston, we've had a problem" situation. I'm starting to wonder if this is some kind of unofficial challenge.

I'm not sure what would be the best maneuver to do, but I noticed that you still have a lot of monopropellant. It should give you additional 50 m/s if you use it first, for a total of 540 m/s. If you do the exact oposite and use all LF first, it is 530 m/s total. But it should be possible to use RCS and nukes at the same time to perform a maneuver. Unfortunately, that probably won't be enough, although it could help a bit.

Otherwise than that, I considered knocking off one of the fuel tanks with Poodle engine, reducing the mass by 5 tonns. It would give more than 600 m/s, but you would have to run the remaining nuke on some 20% throttle or so to maintain control.

[TaxiService]

Hey there,

Thanks for the ideas. I am at work so I will take a look at yours later but I can comment on some ideas right now.

...So you're actually orbiting Duna retrograde? i.e. westwards, opposite the direction of Duna's spin, Ike's orbit, and Duna's orbit around the sun? Ew, that makes life harder. Just out of curiosity, how did that happen? Anyway, water under the bridge at this point.

I forgot to rotate my entry flyby of Duna to prograde after aligning the flyby plane against with Duna's plane, until too late after I burnt it into current orbit (with extra mass of second nuke).

You could also consider a hybrid approach between "send them home in the hitchhiker" and "send a rescue mission": that is, send them home in the hitchhiker with the best burn you can manage from Duna, then launch a "rescue" mission that intercepts them before they get to Kerbin. The rescue mission wouldn't even have to match velocity or dock-- it could whizz past at kilometers per second at a distance of a few thousand km, and simply serve as a RemoteTech relay that would give you control over the ship long enough to do a correction burn. It could be small, cheap, unmanned, and (because really light) have a lot of dV.

In the it's-so-crazy-it-just-might-work department: You could send them home on EVA. A kerbal's EVA pack has a ridiculously overpowered dV of several hundred m/s. Launch them in the ship, then when you run out of rocket fuel, they go EVA and complete the boost. Yes, it would be a hassle (you'd have to do it separately for each kerbal, and you won't have navball or maneuver nodes)... but they'd live forever in legend and song!

...I like the crazy idea of getting off the wild ride with science stuff and rescuing them later. I will have to try it later.

Also, before ditching the lander, use up the rest of monoprop for a few more km/s before the proper escape burn. And plan four nodes: 1. Escape Duna, get Ike flyby. That should take most of your delta-V; Oberth effect at Duna low orbit will be much stronger than at Ike flyby. 2. Get a perfect encounter with Ike, 3. get the right escape from Ike (at periapsis) 4. once out of Ike SOI perfect your Kerbin encounter.

Hmmm, I need to do some maths first on this option that may be difficult for my amateur skills.

"B" path is pointless as it will just reduce your speed relative to Duna and force you to waste more more fuel on escape.

"A" path accelerates you, but in the wrong direction - raising your Sun apoapsis instead of lowering it.

Move Ike 180 degrees so that it's moving towards the back of Duna's path. Then apply "A" style path (flyby "behind" Ike) but escaping Duna in the opposite direction than where it's moving. When it comes to escape, retro/prograde orbit really doesn't matter much, so don't sweat it.

Good catch on my mistake! This could be the easiest idea with having to getting into too much maths.

The safest and most simplest method would just to send a refueling probe. Docking ports, plenty of airbrakes, large heatshield (doesn't need that much ablator) and make it sturdy, and just hurl it at Duna at high speed without waiting for the transfer window, and aerobrake hard. And double check to see if the resupply module will enter in a retrograde orbit so it can link up.

That's interesting idea. I was worrying about saving as much remaining fuel as possible but the refuel probe should not be big in mass.

This is a bit sad because you just need a bit more to get the full ship home (550 m/s with a burn at periapsis).

I tried some Ike slingshoot but the fact that your orbit is retrograde AND inclined by 15° doesn't help at all. Most of the time you end up with orbits that makes your ship crash on Duna or put you into an even more inclined orbit.

Yup, at least the guys in my Eve craft have more than plenty of delta v to go home or even visit/flyby its moon.

What's your dV if you toss that lander and leave it in orbit?

What's the dV of the lander if you put all the fuel in it and leave the "transfer stage" behind?

You have 3 guys and seating for 8, toss away some of that extra space and you may have the dV you need.

1) It will be 818 m/s delta v if getting rid of the lander but zero correction-control in Sun SOI (I use Remote Tech mod).

2) Almost zero delta v of the lander because I drained entire oxidizers in the stock transfer tank with nuke. I used up almost whole fuel of lander in the trip of landing and launching.

Very few options, indeed.

If you do option 2, you can get a super accurate encounter using alexmoons transfer calculator and precise node. when you click on the i next to the delta-v in the calculator, it will give you values you can plug in to precise node and only require very minimal tweaking to get an encounter with kerbin. It is how I do all my encounters.

Good idea. I am going to take a look at it later.

- - - Updated - - -

Um, I just thought of another option, based on your comments. How about this: escape-burn with whole craft directly and dump the transfer stage in mid-course. Then use the monoprop to do corrections into Kerbin's SOI. Once my guys enter the SOI as if they are an asteroid, I launch a rescue vehicle and get them before they escape the Kerbin SOI.

Or I just get the guys off (with science of course) the asteroid and brake them with their eva monoprop into orbit of Kerbin. Then I rescue them later like those free-person contract.

Edited July 15, 2015 by TaxiService
Grammar

[Sharpy]

BTW, Kerbin aerocapture/aerobraking correction: 50km shouldn't hurt you. 55km won't touch you.

Yesterday I whizzed by Kerbin at some 3500m/s through 55km. Didn't even get flames, nor any noticeable slowdown. By all means, 48km should still be survivable (with you shooting out the other side of course)

[Lelitu]

Direct re-entry is usually pretty safe from Duna for a ship consisting of command pod, heatshield and parachutes, in stock 1.0.4 aero.

With a near optimal transfer window, and periapsis for somewhere between 20 and 25km I used about 25% of ablator.

It doesn't look like there's a decoupler on the lander section, so that may be somewhat less safe.

[sdj64]

You might be able to get a few extra m/s by using the last 17 units of oxidizer with the Poodle first, then flipping around and using the nukes. With the poodle and nukes together, are you sure KER isn't lying to you? It does get confused with multiple engines, especially those that burn different fuel mixes. Also boost with the RCS thrusters, that 131 monopropellent shouldn't go to waste, besides, it will make your craft lighter once it's gone.

Remember to always use you lowest ISP fuel first. In this case, that means RCS, then poodle, then nukes.

Of course I might be wrong, quicksave first unless you have some personal rule against it.

[Weywot8]

Upload the save, make it an informal challenge!

But sending rescue missions and rescue-rescue missions is the Kerbal way.

[rofltehcat]

Do you have KAS/KIS? I'd move the radar dish from the transfer stage, then cruise home with the 800 dV option.

[Superfluous J]

All his fuel is just LF with no O, which means the LV-Ns are his only option.

...the only disadvantage being that then he wouldn't be able to control his ship for most of the journey: as soon as he departs Duna's neighborhood, he's incommunicado and therefore uncontrollable (because RemoteTech) until he gets right near Kerbin, which means no mid-course correction burn possible.

1) It will be 818 m/s delta v if getting rid of the lander but zero correction-control in Sun SOI (I use Remote Tech mod).

I will admit I totally missed the use of RemoteTech and lack of oxidizer (and I've not used LV-Ns enough in 1.0.x to remember offhand that they no longer use oxidizer). However, if you have the dV to get home doesn't RemoteTech have a "Do this maneuver node when it comes, even though you won't be in communication range at the time" function?

And even if it doesn't, this will be a good chance to hone the very important skill of nailing a target on the other side of the system with a single efficient burn.

Um, I just thought of another option, based on your comments. How about this: escape-burn with whole craft directly and dump the transfer stage in mid-course. Then use the monoprop to do corrections into Kerbin's SOI. Once my guys enter the SOI as if they are an asteroid, I launch a rescue vehicle and get them before they escape the Kerbin SOI.

Sounds good. It's very close to what I'd try my own self

[heng]

Remember, you can always perform the Scott-Manley-Maneuver :-D That should help.

[TaxiService]

Do you have KAS/KIS? I'd move the radar dish from the transfer stage, then cruise home with the 800 dV option.

The dish? There is only the short-range antenna on the transfer stage and I have almost zero oxidizers left. But see below for the feasible solution.

I will admit I totally missed the use of RemoteTech and lack of oxidizer (and I've not used LV-Ns enough in 1.0.x to remember offhand that they no longer use oxidizer). However, if you have the dV to get home doesn't RemoteTech have a "Do this maneuver node when it comes, even though you won't be in communication range at the time" function?

Brilliant! I totally forgot about the RT queue of timed tasks. I think my delta-v problem will be solved, with dumping the lander and letting RT running the whole burning business.

Remember, you can always perform the Scott-Manley-Maneuver :-D That should help.

Good find!